1. Field of the Invention
The present invention relates to a platinum (Pt) deposition technique for depositing, as electrode material, a platinum thin-film or layer on a silicon wafers generally used in producing oxide thin-film devices or semiconductors, and in particular to a method of forming a platinum thin-film on a silicon wafer substantially without using any glue layer and to a silicon substrate substantially free of any glue layer. Herein, the term "silicon substrate" is used to indicate a silicon wafer, on which a platinum layer is deposited.
2. Description of the Prior Art
Nowadays, the thinning of dielectric, piezo-electric, super-conductive, and magnetic ceramic materials becomes a world-wide tendency in order to meet the requirements of miniaturation, high density integration and functional elevation of electronic ceramic parts or devices. For thinning ceramic materials, single crystal materials such as silicon, MgO, SrTiO.sub.3, LaAlO.sub.3, and sapphire, and polycrystal materials such as alumina and diamond have been used as a substrate.
Among them, silicon single crystal wafers have been most widely used in conventional semiconductor production processes, since they can be readily adopted to fabricate thin-film devices and circuits through substrate design procedures and are much cheaper as compared with other single crystal wafers or poly-crystal diamond wafers.
When those silicon single crystal wafers are used in producing oxide thin-film devices, it is needed to form electrode layer(s) for either connecting parts within the devices each other or connecting the devices and external circuits, and/or for operating the devices.
Aluminum, copper, platinum, conductive oxide (RuO.sub.2), and etc. are used as electrode materials. Aluminum is most widely used as a bottom electrode material in conventional dynamic random access memory (DRAM) devices. For novel devices such as new generation DRAM or ferro-electric random access memory (FRAM) devices, the capacity of which have been raised to 1 G to 4 G bits, however, platinum will be used for the bottom electrode materials when ferro-electric oxide materials of Perovskite structure (BT; BaTiO.sub.3, PZT: PbZr.sub.1-x Ti.sub.x O.sub.3, PLZT: Pb.sub.1-x La.sub.x Zr.sub.1-y Ti.sub.y O.sub.3, BST: Ba.sub.1-x Sr.sub.x TiO.sub.3, etc.) having high dielectric constants or spontaneous polarization characteristics are used as materials for capacitors of those devices instead of conventional SiO.sub.2 /Si.sub.3 N.sub.4 group materials.
Since temperatures for forming the ferro-electric materials are high (700.degree. C.) and exceed the temperature of 500.degree. C., below which temperature aluminum electrodes can be used, platinum electrodes which are thermally and chemically stable are more suitable than aluminum electrodes. Furthermore, it is also possible to obtain thin films having excellent crystallization or orientation if platinum electrodes are used, since nucleation which is very important in a course of forming ferro-electric materials can be initiated more readily on platinum layers than on the other electrode materials.
Examples of well-known methods for depositing platinum thin-film are DC/RF magnetron sputtering, vacuum evaporation, metal organic chemical vapor deposition (MOCVD), and ion plating methods. Among them, the vacuum evaporation method has a disadvantage that the adhesive strength between a silicon wafer and a platinum layer is inferior and the MOCVD method has a disadvantage that impurities such as carbon may be introduced into the platinum layer.
In the DC/RF magnetron sputtering method, if the deposition process is performed under the inert atmosphere as known in the art, hillocks or voids are formed during an annealing process performed after deposition or the other processes such as oxide deposition, since the adhesive strength between the platinum layer and the silicon wafer (SiO.sub.2 /Si) is not good, and may result in a short of circuit and/or the compositional in homogeneity of oxides on the platinum layer. Furthermore, the interface characteristics between oxide and platinum layers are deteriorated.
In order to solve the above problems, there are generally used methods which interpose a material selected from Ti, Ta, TiN, etc. between a platinum electrode layer and a wafer (SiO.sub.2 /Si) to serve as a glue layer or methods which do not use platinum but use conductive oxides (RuO.sub.2) as electrode materials.
When Ti, Ta or TiN is used as a glue layer, the adhesive strength can be increased at the time of deposition. However, there are serious problems that the surface roughness becomes deterioriated since the materials of glue layer defuse into the platinum layer and react with oxygen, thereby forming oxides (TiO.sub.2 /Ta.sub.2 O.sub.3), that functions of electrodes and/or concections of upper and lower parts may be lost because of the formation of voids and hillocks, and that the adhesive strength is weakened at the end because of the loss of adhesive function due to the oxidation of the metals used as a glue layer.
In addition, if a conductive oxide such as RuO.sub.2 is used for an electrode, leakage currents of resultant devices are increased because the electrode surfaces are very rough, and the resistivity of the oxide is higher than that of a platinum-layer. A chemical reaction between the oxide and a ferro-electric oxide thin film may also be caused at the ferro-electric film deposition process performed thereafter.
As noted above, there are many problems in using platinum as electrode material for oxide thin-film devices, but it is the real situation that no solutions for those problems have been reported.